Fri night/Sat/Sun do trend to be more crowded days due to the influx of Florida residents. If you are visiting doing the off season, any park should be a good choice or Downtown Disney. If you are visiting during the busy season, MK will be very crowded during the weekends. Epcot is a good choice, just remember the crowds normally migrate from future world to world showcase throughout the day. I would advise getting to Epcot around 9am, from 9-11am going on the most popular rides in future world (Sorin, Test Track, etc.), then 11-3pm going around world showcase and lunch, and 3-7pm completing the rest of future world's attraction (Sea with Nemo, Imagination, Living with the Land, Universe of Energy, etc.), then 7-9pm dinner, shopping and fireworks. Animal Kingdom is also a good choice, but since it normally closes earlier than the other parks there can be large crowds in the mornings. Studios is a good Friday choice, I would advise getting there early to ride the most popular rides or get fastpasses (Tower of Terror, Toy Story Mania, Rock'n'Roller Coaster). The other attractions normally won't be that crowded. Downtown Disney tends to be most crowded weekend nights (due to dining, the movie theater, and cirque). I am uncertain about the water parks...
